1. Field of the Invention
The present invention relates generally to a novel process of producing styrenic polymers and more particularly the invention relates to a suspension polymerization process of producing styrenic polymer particles having a uniform particle size or of producing foamable styrenic polymer particles having a uniform particle size.
2. Description of the Prior Art
Styrene polymers are conventionally prepared by known processes such as emulsion polymerization and suspension polymerization. And the suspension polymerization process sould be utilized when it is desired to obtain polymers in particulate form as the immediate product. However, as it has been very difficult to obtain those of uniform or predetermined size by conventional process, we have had to seive the resulting polymer particles in a desired size range. Among the polymer particles seived off, smaller ones could be further polymerized to obtain predetermined, large particle of uniform size, as proposed in British Pat. No. 1,255,237 in the name of Sekisui Kagaku Kogyo K.K.
British Pat. No. 1,255,237 discloses a suspension polymerization process for obtaining styrenic polymer particles having a large and uniform particle size by dispersing polymer particles prepared by known processes having a small and uniform particle size into an aqueous suspension solution containing a dispersing agent, adding thereto a monomer containing a polymerization catalyst while stirring thereby absorbing the monomer into the polymer particles which are nuclei in the suspension system with the polymerization proceeding in the interior of the particles.
According to the above method, the polymerization catalyst is added to a dispersion system where the polymer particles are suspended therein after uniformly dissolving the polymerization catalyst into the system all of the monomer used. In such a case, the disclosure is that the monomer is added to the dispersion system continuously or intermittently in an amount not exceeding 1.5 times of the monomer in the dispersion system to the polymer in the dispersion system. By this method styrenic polymer particles having a far more uniform particle size as compared with that obtained in a conventional suspension polymerization method can be obtained, but a problem still remains in that the powdery polymer particles which pass through a 32 mesh sieve (a large portion is fine powdery polymer particles having a particle size of not more than 0.2 mm) are by-produced in an amount of 2 to 3% by weight. Such fine powdery polymer particles have no utilitarian value since the degree of polymerization is very low and impurities such as suspending agent are contained therein. Further, the recovery of these particles during production is difficult and waste water is contaminated therewith. Thus, problems of an increase in the cost due to the need for waste water treatment facilities occur and, if recovered, a problem of disposal of the particles results. Therefore, as the industrial production amount is so great, reduction in the amount of powdery polymer particles formed to a degree of 0.5 to 1.0% by weight brings about a big economical advantage. Further industrial waste materials can also reduced.
As mentioned before, in the method hithertofore reported, the approach is to merely dissolve all of the catalyst in all of the monomer and to add the resulting mixture to a dispersion system. In employing this method the formation of powdery polymer particles cannot be reduced to less than 1% by weight. Further, other conventional techniques, e.g., adding all of the catalyst and then adding the monomer, or adding all of the monomer and then adding the catalyst, also, does not prevent the formation of fine powdery polymer particles. For the reasons above, an improved suspension polymerization method in which the formation of fine powdery polymer particles is prevented and polymer particles having a uniform particle size can be obtained has hithertofore been strongly desired.